10964
J. Phys. Chem. 1992,96, 10964-10967
Destruction of Organhed Organic Monotayers by Oxygen Atoms
Y.Paz, S.Trakhtenberg, and R. "ann* Department of Chemical Physics, Weizmann Institute of Science, Rehovot, 76100, Israel (Received: July 20, 1992; In Final Form: September 8, 1992)
The destruction of organized organic monolayers by an atomic beam of O(3P) was investigated as a function of the surface temperature. Changes in the infrared absorption and wettability were monitored to measure the extent of reaction of the monolayer. Two different monolayer type were used-one consisting of a methyl-terminated 18-carbon chain and the other of a single methyl group, both bound to the surface through a siloxyl bond. The long chain was found to be much more reactive than the short one, and the reactivity was dependent on the change in structure of the monolayer due to variations in temperature. Clear indications for a phase transition in the long-chain monolayer were found.
Introduction Significant progress has been made in recent years toward the development of stable structured monolayer assemblies consisting of long-chained amphiphile~.'-~These assemblies, which form close-packed structures, can be constructed to selectively expose a variety of chemical functionalities at the air-monolayer interface. Such structures have obvious importance in the regulation of heterogeneous reactions and gas-surface energy transfer. The ability to cover substrates with oleophobic and hydrophobic coatings ensure the maintenance of clean surfaces for long times even under moderate vacuum conditions. In addition, these surfaka provide specific alignment toward reactions, and hence they open a new dimension to the study of gas-surface interactions. In the past, we have studied the time of flight and the internal energy content of molecules and atoms scattered from these monolayer^.^" In other laboratories, experiments in which He atoms were scattered from amphiphilic monolayers were able to probe the surface structure of the monolayer and its temperature dependence.' Recently, atoms and molecules were scattered from organic The results resemble those obtained from scattering from organized organic molecules (OOM). Reactions of OOM, both self-assembled (SA) and Langmuir-Blodgett (LB), in solution with KMn0410and in a gas with ozone'' were investigated by various groups. In the present work, results are presented on the reactivity of OOM with a beam of atoms studied by monitoring the OOM. Specifically, the reactions of two organized amphiphilic monolayers, octadecyltrichlomilane (OTS)and methyltrichlomsilane(MTS), with ground-state oxygen atoms O(3P) were studied. Reactions of atomic oxygen with saturated hydrocarbons have been investigated thoroughly in the gas phase for atoms both in For the their ground state 3P12and in the ID ground-stage oxygen, it is known that the barriers for its reaction with primary, secondary, and tertiary hydrocarbon groups to produce OH are 6.9,4.5, and 3.3 kcal/mol, respectively." All three types of reactions are slightly exothermic with AW = -2.3, -7.0, and -10.3 kcal/mol for primary, secondary, and tertiary abstraction, respectively. It has also been established that in the gas phase, the reaction occurs when the O(jP) approaches the C-H bond in the hydrocarbon collinearly. On the other hand, the reaction of O(lD) with hydrocarbons is an insertion with no barrier. The singlet and triplet potential energy surfaces cross when the oxygen atom approaches the C-H bond at 90°."
Experimeutal Section The experimental system consists of a vacuum chamber pumped by a liquid nitrogen trapped diffusion pump. The background pressure in the chamber is typically in the l(r7-Torr region. O(3P) is generated by a 2450-MHz microwave discharge of oxygen flowing in a 6-mm-diameter quartz tube treated with phosphoric acid. The total flow through the tube was about 9.4 X 10l6oxygen molecules/s. On the basis of past studies with this source, the concentration of oxygen atoms in the flow is estimated to be a few percent.
Slides covered with the organic monolayers are held by a temperature-controlled manipulator, facing the oxygen source. The surface temperature is controlled by a Eurotherm 818 temperature controller. The monolayers studied in this experiment were prepared either on glass slides or on glass slides coated with an aluminum layer. The OTS monolayer consists of 18-carbon alkyl chains attached to the surface through siloxyl groups and cross-linked through S i U S i bonds. In the case of MTS the methyl group is attached directly to the siloxyl group. The surfaces were probed by two techniques, contact angle measurements (wettability) with water and with bicyclohexyl (BCH) and various IR absorption methods. The wettability measurements are sensitive to structural changes in the monolayer, as well as to chemical changes in the outer functional group. Such changes can be, for example, the formation of hydrophilic C-OH bonds. Typically the contact angles measured with OTS are 113O and 5 l 0 with water and BCH, respectively, whereas for MTS the corresponding contact angles are 93O and 4 6 O . All contact angle measurements were done at room temperature, and the repeatability of the measurements was within 2O. Three types of IR absorption methods were applied. External multireflection Fourier transform infrared (EXRIR)spectroscopy was employed for measuring the degree of order of the monolayers as a function of the surface temperature and its change due to the reaction. When the EXRIR is applied, the OOM is adsorbed on a slide coated by a metal film. The electromagnetic field parallel to the surface is canceled by the image field, and only vibrational modes with transition dipole perpendicular to the surface can be detected.19 The second method is the single-pass direct absorption IR spectroscopy (DAIR) on glass slides. Here only modes with transition dipole moment parallel to the surface absorb light, while those with the dipole moment normal to the surface are inactive. Hence this second type of measurement is complementary to the EXRIR method. The third technique is the attenuated total reflection (ATR) method in which the OOM is adsorbed on a 3-mm silicon crystal? The IR beam is introduced through the side of the crystal at an angle close to critical. The absorption as measured by monitoring the reflected beam, is due to the evanescent wave penetrating through the surface of the crystal. Here the absorption is not very sensitive to the alignment of the monolayer relative to the surface. ReSUltS
Figure 1 presents a typical room-temperature grazing angle FTIR (EXRIR) spectrum of an OTS monolayer chemisorbed on aluminum. The ratio of the intensities of the CH3 peaks at 2878 (symmetric stretch) and 2966 cm-'(asymmetric stretch) relative to the intensities of the CH2at 2852 (symmetric stretch) and 2920 cm-' (asymmetric stretch) is a sensitive indication for the high degree of order in the monolayer and its orientation relative to the s ~ r f a c e .Only ~ ~ ~in~highly ordered OTS the peaks corresponding to the single methyl group have the same intensities as those result from the 17 methylene groups. Figure 2 presents the ratio of the intensities of the bands corresponding to the CH3and
0022-3654/92/2096- 10964$03.00/0 0 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 26, 1992 10965
Destruction of Organized Organic Monolayers
8 0.000
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Figure 4. Attenuated total reflection (ATR) IR spectrum of OTS ad-
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Figure 1. Typical room-temperature grazing angle FTIR (EXRIR)
spectrum of OTS monolayer adsorbed on aluminum. 1.6
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surface normal. The spectrum of the monolayer before exposure to oxygen is shown in the lower curve. The peak at 2960 cm-I corresponds to the methyl asymmetric stretch, while those at 2925 and 2850 cm-I correspond to the absorption of the methylene asymmetric and symmetric stretching, respectively. The spectra of the OTS after one side of the prism was exposed for 30 min to the oxygen beam (middle curve) and then the second side of the prism was also exposed (upper curve). For clarity of presentation the spectra are shifted on the vertical scale. I
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Figure 2. The ratio (R)between the band intensities of the CH3 vs the
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Figure 5. Change in absorption in the EXRIR spectrum of OTS as a function of surface temperature during the reaction. The points assigned with and correspond to the absorption of the CH3 symmetric and asymmetric stretching respectively. The points assigned with 0 and 0 correspond to the CHI symmetric and asymmetric stretch, respectively.
D
9
Wavenumber (c")
Figure 3. Direct absorption FTIR (DAIR) spectrum of OTS showing the CHI and CHI peak before (solid line) and after (dashed line) the reaction. The surface temperature during the reaction was 25 OC (A) and 110 OC (B).
the CH2 asymmetric stretches in the EXRIR spectrum as a function of the surface temperature. The change in the ratio is reversible in temperature up to 180 OC. Hence, the change cannot involve the total collapse of the structure of the OOM. In Figure 3 the DAIR spectrum of OTS is shown before and after the reaction. The surface temperature during the reaction was 25 and 110 OC for parts A and B, respectively. It is evident that while for the surface, at low temperature, the CH3 peak is depleted due to the reaction, for the high temperature, the CH2 groups are reacting. When MTS is monitored by the same method and after the surface is exposed for the same time to the oxygen beam, we find that the decrease in the methyl signal is only 20% of that observed in OTS. The ATR spectra of OTS monolayer as a function of the reaction time are presented in Figure 4. The monolayer was prepared on a silicon prism, and its spectrum before the exposure to the oxygen beam is presented in the lower trace. The ratio between the peaks corresponding to the CH3 asymmetric stretch and the CHI asymmetric stretch reflects simply the ratio in the 'concentration" of these two chromophores, namely, 1:17. In the middle trace the spectrum is shown for the same monolayer after
one side of the prism was exposed to the oxygen beam for 30 min at 25 OC. The spectrum in the upper trace was obtained after the second side was exposed for the same time length. The disappearance of the methyl peak is an indication of its reactivity. In Figure 5 the changes in the EXRIR absorption peaks is shown as a function of the surface temperature during the reaction of OTS with the atomic beam. The monolayer was exposed for 25 s to the oxygen beam at all temperatures. While at low temperatures, the intensity of the absorption corresponding to the methyl group decreases, that of the methylene group increases. At temperatures around 100 OC all the peaks decrease at the same rate; however, at higher temperatures, due to the disordered structure, the signal corresponding to the methyl increases. These results indicate that at low temperatures the outer methyl groups are mainly reacting, while at higher temperatures mainly the methylene groups participate in the process. Although we made efforts to detect the formation of C-0 bonds following the reaction, no indication for such bonds was found in any of the IR techniques applied. Information on the reactivity can also be extracted from wcttability measurements, by monitoring the contact angle between the surface covered with the OOM and various liquids. In these studies when water is used as the wetting liquid, one is probing the hydrophobic characteristics of the surface due to the dense coverage of the hydrocarbon. However, when BCH is applied, one is sensitive to the oleophilic nature of the surface due to its close packing organization. Figure 6 presents the measured contact angles with water as function of the exposure time to the OCP) beam for OTS and MTS surfaces. The contact angle drops faster for the OTS than it does for the MTS surface. Figure 7 presents the contact angles for a set of OTS monolayers adsorbed on A1 or directly on glass slides. All surfaces were exposed for 25 s to
10966 The Journal of Physical Chemistry, Vol. 96, No. 26, 1992
Paz et al. respectively) were exposed to the triplet oxygen for a short time,
the changes in l T I R spectra prove that the methylene group is '30-----1 reacting even during the first few seconds of exposure.
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, 8 12 16 20 24 28
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Figure 6. Contact angle following the reaction, measured with water on OTS (A)and MTS (0)surfaces as function of the exposure time of the surface to the O(") beam. 951 0
The above description also explains the correlation between the reactivity of the monolayer and the change in its structure due to heating. This change of structure can be noticed by an abrupt decrease in the CH3/CH2peak ratio in the EXRIR spectrum and implies that on average the chains are less constrained in their motion relative to the normal to the substrate. This motion, which can be no more than several degrees, enables the penetration of atomic oxygen and the reaction with the methylene groups. Indeed, the DAIR signal of the methylene group after short-time exposure to the OCP) decreases, when the temperature is kept above ca. 80 OC, whereas reaction at lower temperatures affects the signal only slightly (Figure 3). The correlation between the change in structure between 80 and 110 OC and the reactivity of OTS gives rise to questions about the nature of this structural change. On the basis of X-ray diffraction studies and in view of the easy access to the methylene group, it seems that the chains are going from all-trans configuration to a cis-trans mixture. Only 1.2 X 10-4 of the oxygen atoms, in the thermal beam,have enough energy to cross the barrier of 6.9 kcal/mol and react with the methyl group. Including the geometrical factor and assuming that the O(3P) is about 10% of the beam, for the given flow rate, only about 8 X lo8 atoms/(s an2)reach the surface with enough kinetic energy to cross the barrier. In the monolayers, there are 5 X l O I 4 molecules/an2. Therefore, it should take about 6 X lo5 s for 10% of the monolayer to react. Instead, we observed that this portion of the monolayer is reacting in 20 s. Hence we must conclude that almost all atoms impinging on the surface do react independent of their initial kinetic energy. The question is why and how the methyl group in OTS reacts, while on the basis of gas-phase studies there is a high bamer for the proms. In addition one has to realize that the same mechanism inducing the reactivity in OTS does not affect the reactivity of the MTS. Assuming that the barrier can be crossed given the system enough time, one expects that if the residence time of the oxygen atom is long, reactions would occur, while for surfaces from which the atoms scattered almost elastically, the reactivity will be lower. Following this idea, the reactivity of the OTS results from the long lived collision complex between the O(3P)and the monolayer. It is expected, on the basis of former studies," that the OTS will absorb most of the oxygen kinetic energy, thereby allowing the atom to reside for a long time near the surface. Since there are fewer low-frequency degrees of freedom in the MTS monolayer, the resident time of the oxygen will be much shorter on the MTS surface, resulting in low reactivity. The problem with this model is however that due to the absorption of the kinetic energy by the OTS, less energy remains in the reactive coordinate and the probability to cross the barrier is also reduced. This difficulty may be resolved if one r e a l i i that during the long time the atom spends on the surface, curve crossing may occur to the singlet potential energy surface, on which no barrier exists. This type of process has been observed in the reaction of O('P) with hydrocarbon clusters.22 To obtain a complete insight on the reactivity of the OOM with O(3P), the gas-phase reaction products have also to be monitored. Studies in this direction are under way and together with the present work may provide new insight into the reactivity of adsorbed organic molecules.
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Figure 7. Contact angle measured with water (A) and bicyclohexyl (B) following reaction with OOP) at various surface temperatures. Full dots correspond to OTS adsorbed on glass, while open circles correspond to OTS adsorbed on glass coated with aluminum.
the O(3P) beam but at varied surface temperatures. The abrupt decrease in both H 2 0 and BCH CA at about 90 "C indicates a steep increase in the reaction rate at that temperature. The results point to the following conclusions: (a) From DAIR and CA measurements it seems that the methyl group reacts with the OTS monolay about 5 times faster than it does with the MTS methyl. (b) As the surface temperature increases, most of the reaction in the OTS surface occurs between the oxygen atom and the CH2group. (c) Change in contact angle as a function of the surface temperature during the reaction relate to the change of the peak ratio in the EXRIR spectrum and indicate a phase transition. This conclusion is in agreement with recent results of X-ray diffraction measurements.21
Discussion The results p r w n t two main effects. The methyl group in the OTS reacts faster than in the MTS and a phase transition in the OTS dramatically affects its reactivity. In what follows each of the above conclusions will be substantiate and rationalized. To understand the relation between the structure of the monolayer and its reactivity, it is essential to determine whether the methylene or the methyl is the dominant reactant. Although the CH3is the outer group, the energy bamer for its reaction (based on gas-phase experiments) is by far larger than that for the CH2 group. Therefore, the ability of the oxygen atoms to penetrate between the chains is the main factor controlling the branching of the reaction between these two functional groups. The data shown above suggest that at room temperature it is the methyl group which is the first to react. The sharp decrease of both symmetric and asymmetric CH3 peaks (ca. 45% decrease in absorption for exposure time of 25 s; Figure 5) is strong evidence for this statement. From the EXRIR spectra it is evident that at this stage of the reaction, the CHI signal increases due to the disorder caused by the reaction of the methyl group. Thii increase is most pronounced in the asymmetric mode, reflecting largeamplitude motions in the direction perpendicular to the plane defined by the C-C-C skeleton. As the reaction proceeds, the penetration of oxygen becomes easier, and the methylenes begin to participate in the reaction. Indeed, when less ordered OTS monolayers (ca. 11 1O and 4 6 O contact angles in H 2 0 and BCH,
Acknowledgment. We thank Dr.S.&hen for a critical reading of the manuscript. This work was partially supported by the Foundation for Basic Research administrated by the Israel Academy of Sciences and by the MINERVA foundation. References and Notes (1) For example: Troughton, E. B.; Bain, C. D.; Whitesides, G. M.; Nuzzo, R. G.; Allara, D. L.; Porter, M. D. Lmngmuir 1988, 4, 365 and references therein. (2) Maoz, R.; Sagiv, J. J . Colloid Inter/oce Sci. 1984, 100, 465. (3) Ulman, A. Introduction to Utrarhin Organic Films; Academic Press: New York, 1990.
J. Phys. Chem. 1992,96, 10967-10970 (4) Cohcn, S.;Naaman, R.; Sagiv, J. Phys. Rev. Lett. 1987,58,1208; J. Chem. Phys. 1988,88,2757. (5) Cohen, S. R.; Naaman, R.; Balint-Kurti, G. G. Chem. Phys. 1989,134, 119. (6)Paz, Y.;Naaman, R. Chem. Phys. Lett. 1990,172,120. (7) Chidsey, C. E. D.; Liu, G. Y.; Rowntre, P.; Scoles, G. J. Chem. Phys. 1989,91, 6926. ( 8 ) Saecker, M. E.; Govoni, S. T.; Kowalski, D. V.; King, M.E.; Nathanson, G. M.Science 1991,252, 1421. (9) Kenyon, A. J.; McCaffery, A. J.; Quintella, C. M.; Zidan, M. D. Chem. Phys. Lett. 1992, 190, 55. (10) Maoz, R.; Sagiv. J. Thin Solid Films 1985,132, 135. (1 1) Maoz, R. PLD. Dissertation, Weizmann Institute, 1985. (12) Andresen, P.; Luntz, A. C. J . Chem. Phys. 1980, 72,5842. (13) Yamazaki, H.; Cvetanovic, R. J. J . Phys. Chem. 1964,41, 3703.
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1980,73,6351. (11) Luntz, A. C. J . Chem. Phys. 1980, 73, 1143. (18) Aker, P. M.; OBrien, J. J.; Sloan, J. J. J. Chem. Phys. 1986,84,745. Aker, P. M.; Niefer, B. I.; Sloan, J. J.; Heydtmann, R. J. Chem. Phys. 1987, 87, 203. (19) Gun, J.; IscoviCi, R.; Sagiv, J. J . Colloid Interfwe Sci. 1984,101,201. (20) Cohen, S.R.; Naaman, R.; Sagiv, J. J. Phys. Chem. 1986,90,3054. (21) Tippmann-Krayer, P.; Kenn, R. M.; Mohwald, H. Thin Solid Films 1992,210l2I1,511. (22) Rudich, Y . ;Lifson, S.;Naaman, R. J . Am. Chem. Sot. 1991, 113, 7011.
Use of ESCA Valence Bands To Infer Structural Information about the Molybdenum Phase In Supported Molybdenum Catalysts Joseph N. FiedorJ Andrew Proctor2 Marwan Houalla,+Peter M. A. Shemood,* Francis M. Mulathy,* and David M. Hercules*.+ Department of Chemistry, University of Pittsburgh at Pittsburgh, Pittsburgh, Pennsylvania 15260, Department of Chemistry, Kansas State University, Manhattan, Kansas 66502, and Department of Chemistry, University of Pittsburgh at Bradford, Bradford, Pennsylvania 17601 (Received: July 27, 1992; In Final Form: September 18, 1992)
X-ray photoelectron spectroscopy (XPS, ESCA) valence band spectra were used to infer information about the molybdenum phase in standard Mo compounds such as sodium molybdate and ammonium heptamolybdate and in Mo/A1203catalysts prepared by equilibrium adsorption and incipient wetness impregnation methods. ESCA valence band measurements can distinguish between tetrahedrally coordinated Mo (sodium molybdate) and octahedrally coordinated Mo (ammonium molybdate). Mo ESCA valence band results show that Mo/A1203catalysts prepared by equilibrium adsorption at pH 2.2 and Mo/A1203 catalysts prepared by incipient wetness impregnation contain primarily octahedral Mo.
toward changes in the symmetry of the element investigated. For Supported molybdenum (Mo) catalysts in their oxidic form are example, both tetrahedral monomeric Mo6+ and octahedral often prepared by pore volume impregnation of a high surface polymeric Mo6+exhibit the same Mo 3d5,2binding energy value area support (A1203,T i 0 3 with aqueous molybdate solution (-232.5 eV). followed by drying and calcination. Altem ively, the catalysts In principle, because of their sensitivity to chemical bonding, can be obtained by equilibrium adsorption of% o oxyanions l from ESCA valence bands may reflect changes in the symmetry of the solution.' However, regardless of the preparation method used surface species. Recently, Sherwood et a1.I8 showed that the and provided that the Mo content is below nominal monolayer valence band for many metals with oxygen ligands can be predicted awerage of the support, oxidic catalysts usually contain tetrahedral by theoretical Xa!calculations. This provided a valuable tool for monomeric Mo, octahedral polymeric Mo species, or both. The the interpretation of valence band spectra. A survey of theoretical distribution of these species is a function of various preparation and experimental results reported by Sherwood et al.I9 indicates parameters (pH of the Mo-impregnating solution, calcination that the valence band spectrum of molybdenum trioxide, where temperature, nature of the support used, and Mo-precursor solution). The nature of supported Mo species in the oxidic catalyst Mo is octahedrally coordinated, differs from that of sodium and their abundances often determine the catalytic properties of molybdate where Mo is tetrabedrally coordinated. This suggests the active form. It is therefore of interest to carry out a detailed that Mo valence band spectra may be used to distinguish between tetrahedrally and octahedrally coordinated Mo. The purpose of characterization of oxidic supported Mo catalysts. Several techniques, such as Raman,'-I2 diffuse reflectance the present work is to assess the potential use of ESCA valence s p e c t r ~ s c o p y Fourier ~ ' ~ ~ ~transform ~ infrared spectroscopy (FTband measurements for determining the symmetry of Mo in IR),I4and extended X-ray absorption fine structure (EXAFS),1s-'7 Mo/A1203 catalysts and in standard Mo compounds. have been employed to investigate the structure (symmetry) of Experimental Section supported Mo oxidic catalysts. However, in many instances, the Materials. Ammonium heptamolybdate ((NH4)6M07024) results from a single technique were inconclusive. This is due (Fischer Scientific) and sodium molybdate (NaMo04) (Alfa) were mainly to the inherent limitations of each technique. Therefore, used as the standards for octahedrally and tetrahedrally coorthere is always interest in finding different means for monitoring dinated Mo, respectively. Harshaw y-alumina (surface area = the structure of the oxidic Mo catalysts. 170 m2/g) was used for the preparation of the physical mixtures ESCA is an established technique for monitoring changes of with sodium molybdate and ammonium heptamolybdate as well oxidation state for a given element. This is due to significant for the preparation of the Mo/A1203catalysts. The composition binding energy shifts for core level lines on reduction or oxidation. of the physical mixture was adjusted to obtain ESCA intensity However, the technique of monitoring core levels is less sensitive similar to those of industrially used Mo/A1203 ratios ZMo3d/ZA12p catalysts. Details of the equilibrium adsorption and incipient University of Pittsburgh at Pittsburgh. wetness preparation methods used to prepare the Mo/A1203 8 Kansas State Universitv. University of Pittsburgh at Bradford. catalysts employed in this study have been published previously.~J3 QQ22-3654/92/2096-lQ967%Q3.QQ/O0 1992 American Chemical Society
